1. Field of the Invention
The present invention relates to amplifier circuits and, in particular, to broadband amplifier circuits having high dynamic characteristics and, in particular, to amplifiers having programmable amplification.
2. Description of the Related Art
In many fields of signal processing and, in particular, in the field of video amplifiers, there is demand for an amplifier having a very broad band. Typical requirements for an amplifier in the video field are in a transmission band of 50 to 860 MHz. Additionally, amplifiers of this kind should have a low noise ratio and a high linearity. Put differently, this means that non-linear mixing products of the amplifier remain below a specified threshold value. The so-called third order inter-modulation or the so-called “third order intercept point” is a parameter of this.
Furthermore, a mixer is typically downstream of such a broadband amplifier in an input stage of, for example, a television receiver. Mixers most often comprise a certain predetermined input dynamic range. This means that the mixer, on the one hand, requires minimum modulation, i.e. an input signal having a predetermined minimum power, in order for it to operate correctly. Even more important with mixers is the requirement that the input signal into the mixer must not exceed a predetermined maximum power or a predetermined maximum level. If the input signal into the mixer, i.e. the output signal of a broadband amplifier, had too high a level, i.e. if the amplifier amplified to too high an extent, the mixer would be overdriven, which would result in undesired mixing products which may even cause a total failure of the system but would at least severely reduce the signal/noise ratio after the mixer.
Typical solutions for this problem have been to use a broadband amplifier comprising a chain circuit of individual amplifiers, wherein the first amplifier in the chain circuit usually comprises a low noise ratio and has a high amplification, whereas the last amplifier in the chain comprises a small amplification and thus a high noise ratio which, due to the large signal useful level, does not make a real difference.
Thus, the requirement for a minimum level for the mixer is ensured when the overall amplification of the chain circuit having individual amplifiers is set to be sufficiently high.
In order to govern the second problem, i.e. in order to ensure that the mixer is not overdriven, a switchable attenuation member attenuating to a greater or lesser extent depending on the output signal from the amplifier is placed downstream of such an amplifier to obtain the output signal of the entire amplifier/attenuation member assembly at a level which is within the permissible “level corridor” determined by a downstream mixer.
This circuit, however, has several problems. On the one hand, a signal is often amplified in the beginning and subsequently, when the level is too high, attenuated again. The result is a double signal processing of a signal, which is problematic at least as regards the noise introduced, since every signal processing introduces noise into the signal. Furthermore, the chain-connected amplifiers must be designed with care in order to be able to fulfill high linearity requirements.